Maintain Healthy Soil with Crop Rotation

You can promote healthy soil in your garden and cut down on plant disease by practicing a three-year cycle of crop rotation with eight plant families.

| February/March 2010

One of the rules of good organic gardening is to rotate plant families from one season to the next, as best you can, so related crops are not planted in the same spot more often than every three years or so. The purpose of crop rotation is to help maintain the balance of nutrients, organic matter, and microorganisms necessary for healthy soil. Of these three, the invisible world of soil-dwelling micro-creatures is the one that most benefits from crop rotations.

Take potatoes, for example. In the course of a season, the fungi that cause scabby skin patches may proliferate, along with root-killing verticillium fungi (which also damage tomatoes and eggplant) and tiny nematodes that injure potatoes. If you plant potatoes again in the same place, these pathogens will be ready and waiting to sabotage the crop. Rotating the space to another unrelated crop deprives the potato pathogens of the host plant they require. Most pests and diseases can damage plants of the same botanical family, but cannot hurt unrelated crops (see “Rotate Your Families: The Nine Main Groups,” below).

What if you don’t follow a crop rotation plan? Field trials in Connecticut and Europe indicate that your potato production will quickly fall by 40 percent, mostly due to disease. According to a seven-year study from Ontario, you could expect similar declines if you planted tomatoes in the same place over and over again. Compared to eight different rotations with other vegetables or cover crops, continuous tomatoes consistently produced the lowest yields. Snap beans that are not rotated will turn into paltry producers, too. In a recent study from Cornell University, snap bean production doubled when beans were planted after corn rather than after snap beans.

In addition to interrupting disease cycles, rotating crops prevents the depletion of nutrients. For example, tomatoes need plenty of calcium the same way beans and beets crave manganese. But the exact benefits of effective rotations vary with crop sequence. Broad-leafed greens are great for suppressing weeds, and the deep roots of sweet corn do a good job of penetrating compacted subsoil. Nitrogen-fixing legumes often take no more nitrogen from the soil than they replace, and their presence stimulates the growth of beneficial soil microorganisms. But in some situations, the “rotation effect” defies easy explanation. For example, we don’t know precisely why potatoes tend to grow well when planted after sweet corn, but they do.

The subject of crop rotation can get complicated so fast that it’s no wonder we are tempted to cheat. What if your garden is like mine — a collection of a dozen permanent beds that are planted with 20-plus different crops in the course of a growing season? Not using rotations would be unwise. When researchers at Pennsylvania State University tracked early blight of tomatoes grown in the same place for four years, early-season infection rates (measured when 5 percent of fruits turned red) went from 3 percent in the first year to 74 percent in the third. When they tried the same monoculture maneuver with cantaloupes, symptoms of alternaria blight appeared earlier and earlier with each passing season.

Questioning the Rules

Some organic gardeners point out that crop rotation guidelines developed for farmers don’t really fit home gardens. On farms, crop residue is either plowed under or left on the surface to decay, which means the soil receives large infusions of a single type of plant material. Gardeners are more likely to pull up and compost spent crops, and to dig in compost or other soil amendments between plantings, which replenishes nutrients and invigorates the soil food web in an extremely diversified way. Biodegradable mulches introduce more considerations: If you heavily mulch your potatoes with straw, shredded leaves, grass clippings or all three, certainly it makes sense to factor those forms of organic matter into your rotation plans.

4/19/2016 10:48:21 AM

I want to plant sweet corn where I had tomatoes last year. Is this ok, or should I plan on a different spot? Thank you.

2/25/2015 8:58:58 AM

so o was think what was i was going to do on my science i don't know what to do can some body tell me what to do please.

6/17/2014 4:45:54 AM

This is very informative. Thanks for sharing.

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